SKD61 steel is a high-performance hot work die steel specifically designed for applications such as hot forging, die casting, and extrusion dies. Thanks to its high-temperature strength, excellent thermal fatigue resistance, and dimensional stability, it is highly suitable for tools subjected to repeated thermal cycles.
SKD61 achieves a good balance between hardness retention, machinability, and compatibility with surface treatments (e.g., nitriding, PVD coating), which helps significantly extend die service life, enhance process predictability, and reduce maintenance frequency and costs.
Although these grades have similar overall performance, slight differences in chemical composition and heat treatment response may affect the die's performance under high-temperature operating conditions. For cross-border projects or alternative selections, it is recommended to conduct full verification based on specific process conditions.
C: Provides hardness and wear resistance through martensitic transformation; influences secondary hardening.
Si: Strengthens the ferrite matrix and helps resist temper softening.
Mn: Improves hardenability and contributes to overall toughness.
Cr: Enhances hardenability, corrosion resistance, and high-temperature strength.
Mo: Increases red hardness, secondary hardening capacity, and thermal fatigue resistance.
V: Forms fine carbides, improving wear resistance and grain refinement.
The relatively high Cr and Mo contents ensure good dimensional stability and thermal fatigue resistance under repeated high-temperature cycles.
Vanadium carbides can improve wear resistance on critical die surfaces.
Silicon and manganese help optimize temper stability, reducing the risk of softening during long-term use.
After quenching, SKD61 steel forms a tempered martensitic structure with uniformly distributed alloy carbides, which is the basis for its high-temperature strength and thermal fatigue resistance. The main carbide types include:
M₆C: Main elements W and Mo; promotes secondary hardening and maintains high-temperature hardness.
MC: Main element V; refines grains and improves wear resistance.
M₂₃C₆: Main element Cr; enhances high-temperature strength and temper softening resistance.
After quenching, it usually contains a small amount of retained austenite (<3～5%). Although retained austenite slightly reduces hardness, its transformation during service helps compensate for minor stresses in thick sections, thereby improving dimensional stability.
Reduced Inclusions: Minimizes non-metallic inclusions, which are common initiation sites for microcracks under thermal cycles.
Improved Fatigue Life: A cleaner microstructure supports enhanced resistance to thermal cracking and thermal fatigue.
Enhanced Polishing Performance: Fewer inclusions and uniform carbide distribution improve surface finish, which is particularly important for optical-grade or high-gloss dies.
Engineering Significance: For die designers and engineers, SKD61 produced via the ESR process offers higher reliability in long-term production and consistent high-quality surface finish, thereby reducing maintenance and downtime.
Austenitization: Heating to 1020～1080℃ to dissolve alloy carbides and form a uniform austenitic matrix.
Quenching: Using air cooling, oil cooling, or vacuum cooling to transform austenite into martensite, thereby achieving high hardness and heat resistance. The retained austenite content is affected by cooling rate and section thickness.
Tempering: Usually performed 2～3 times to precipitate fine alloy carbides, relieve internal stress, enhance toughness, and reduce the formation of microcracks.
Engineering Significance:
Multiple tempering cycles optimize the balance between hardness and toughness, which is crucial for dies subjected to thermal cycles and high mechanical loads.
Properly controlled heat treatment reduces the risk of soft spots caused by tempering and ensures dimensional stability, especially for complex or thick-walled section dies.
Density: ≈7.8g/cm³; affects handling, fixture design, and structural support.
Thermal Conductivity: 25～30W/m·K; influences heat dissipation and cycle time in high-speed forming.
Elastic Modulus: 210GPa; determines stiffness and resistance to load deformation.
Coefficient of Thermal Expansion: 11.5×10⁻⁶/℃; guides the allowable range for dimensional changes in thick-walled sections.
SKD61 steel is specifically designed for high-temperature die applications and can maintain both hardness and toughness under repeated thermal cycle conditions:
Red Hardness: Effectively retains hardness in the range of 600–650°C, ensuring dimensional stability and consistency in processing/forming performance.
Thermal Fatigue Resistance: The combination of tempered martensitic structure and fine alloy carbides can inhibit the initiation and propagation of thermal cracks during thermal cycles.
Engineering Significance: For die designers, this means SKD61 dies can handle high-speed forging, die casting, and extrusion processes without premature softening or surface degradation.
Thermal Cracking: Initiates from surface defects or inclusions during rapid heating.
Surface Spalling/Chipping: Localized detachment of the hardened layer due to repeated thermal cycles.
Oxidation/Scaling: Reduces surface integrity when exposed to high temperatures or corrosive environments for extended periods.
Mitigation Strategies:
Control heat treatment and tempering to balance hardness and toughness.
Adopt ESR remelting to reduce thermal cracking caused by inclusions.
Design a reasonable cooling system to reduce local overheating and stress concentration.
Annealing: Furnace cooling at 780～820℃ to eliminate internal stress and improve machinability; hardness is approximately 220–250 HB.
Stress Relieving: Performed after rough machining to reduce the risk of subsequent heat treatment.
Austenitization: 1020～1080℃; heating should avoid overheating to prevent grain coarsening.
Quenching: Using oil cooling, air cooling, or gas cooling; the cooling rate must be controlled to prevent quenching cracks.
Tempering: Usually conducted 2～3 times at 540～560℃, resulting in a final hardness of 48～52 HRC.
Engineering Tip: Pre-machining dimensions should account for dimensional changes during quenching and tempering. Precise control of temperature, time, and cooling method can ensure performance consistency.
Nitriding: Forms a high-hardness diffusion layer, significantly improving wear resistance; typical layer depth is 0.2～0.4mm with minimal dimensional deformation.
PVD/CVD Coating: Such as TiN and CrN, which can reduce friction, minimize adhesive wear, and enhance thermal cracking resistance.
Composite Treatment: Nitriding + PVD can exert synergistic advantages in high-cycle forging and die casting dies.
Reasonable fixture design to reduce warpage during heat treatment.
Reserve 0.3～0.5mm allowance for final grinding.
Use light grinding or polishing after heat treatment to achieve the final dimension.
Milling and Turning: Use coated cemented carbide or cermet tools; rough machining cutting speed is 80–120 m/min, and finish machining is 120～180m/min.
Cutting Depth Control: Avoid one-time heavy cutting; adopt multiple passes.
Grinding: Use CBN or alumina grinding wheels with sufficient cooling after heat treatment.
Cooling Method: High-pressure cooling or mist cooling helps dissipate heat and extend tool life.
Gradually use sandpaper from P240～P400 to P800～P1200, and finally use 6～1μm diamond paste or colloidal silica.
Avoid local overheating to prevent orange peel or micro-scratches.
Welding: Use compatible nickel-based or stainless steel die steel welding wire; preheat to 200～300℃.
Post-weld Treatment: Perform stress relieving or tempering to restore performance.
Subsequent Grinding and Polishing: Restore dimensions and surface quality.
Typical Applications: Hot forging dies, die casting dies (aluminum, zinc alloys), extrusion and hot stamping dies.
Case 1: High-Cycle Forging Dies After a forging factory replaced H11 dies with SKD61:
Die service life increased by approximately 30%.
Thermal fatigue cracks were significantly reduced.
Dimensional accuracy was maintained over 500,000 cycles.
Case 2: Aluminum Alloy Die Casting After switching from H13 to SKD61, the oxidation and wear of the die cavity surface were reduced, the scrap rate decreased, and the maintenance cycle was extended.
SKD61 steel is a high-performance hot work die steel with excellent thermal fatigue resistance, red hardness, and dimensional stability. Its tempered martensitic structure with uniformly distributed CrMoV carbides, especially in the ESR-refined version, endows it with higher wear resistance, polishability, and die life. Through reasonable chemical composition control, heat treatment, and surface engineering (e.g., nitriding, PVD/CVD), its hardness, toughness, and fatigue life can be further improved.
For engineers, designers, and purchasers seeking high-temperature durability, thermal fatigue resistance, and long-term precision, SKD61 is a reliable choice superior to SKD6 or H11.
